Ryong Ha

Ferroelectric Nonvolatile Nanowire Memory Circuit Using a Single ZnO Nanowire and Copolymer Top Layer

IO N Nanowire-based fi eld-effect transistors (FETs) and diodes have been continuously studied along with a great variety of semiconductor nanowires (NWs), including Si, Ge, SiGe, GaN, InP, and ZnO. [ 1–8 ] Among all the NW materials, ZnO appears to have relatively good metal electrode–semiconductor contact, which improves the device fabrication yield. [ 7–17 ] Therefore, using a long ZnO NW it may be possible to realize one-dimensional (1D) NW electronics, which may contain a FET, [ 7 , 10–14 ]

Photostable Dynamic Rectification of One-Dimensional Schottky Diode Circuits with a ZnO Nanowire Doped by H during Passivation

For the first time, we demonstrated photostable and dynamic rectification in ZnO nanowire (NW) Schottky diode circuits where two diodes are face-to-face connected in the same ZnO wire. With their properties improved by H-doping from atomic layer deposited Al(2)O(3) passivation, our ZnO NW diode circuits stably operated at a maximum frequency of 100 Hz displaying a good rectification even under the lights. We thus conclude that our results promisingly appoached one-dimensional nanoelectronics.

Long single ZnO nanowire for logic and memory circuits: NOT, NAND, NOR gate, and SRAM

We demonstrate logic and static random access memory (SRAM) circuits using a 100 μm long and 100 nm thin single ZnO nanowire (NW), which acts as a channel of field-effect transistors (FETs) with Al2O3 dielectrics. NW FETs are thus arrayed in one dimension to consist of NOT, NAND, and NOR gate logic, and SRAM circuits. Two respective top-gate NW FETs with Au and indium-tin-oxide (ITO) were connected to form an inverter, the basic NOT gate component, since the former gate leads to an enhanced mode FET while the latter to depletion mode due to their work function difference. Our inverters showed a high voltage gain of 22 under a 5 V operational voltage, resulting in successful operation of all other devices. We thus conclude that our long single NW approach is quite promising to extend the field of nano-electronics.

ZnO nanowire and mesowire for logic inverter fabrication

We report on a ZnO-based logic inverter utilizing two field effect transistors (FETs), whose respective channel has different wire-diameters under a top-gate dielectric of poly-4-vinylphenol. One FET with nanowire (160 nm) channel displayed an abrupt drain current (ID) increase and fast ID saturation near its positive threshold voltage (Vth) while the other FET with mesowire (770 nm) showed a thin-film transistor-like behavior and a negative Vth. When the nanowire and mesowire FETs were, respectively, used as a driver and a load, our inverter demonstrated an excellent voltage gain as high as 25 under a supply voltage of 20 V.

ZnO nanowire transistor inverter using top-gate electrodes with different work functions

ZnO-nanowire field effect transistors (FETs) with a top gate Al2O3 dielectric and different metal electrodes were fabricated to form a low voltage electrical inverter. Two FETs with Pd and Ni/Ti gates whose respective work functions are so different as 5.3 and 4.3 eV were chosen to play as driver and load, since such different work functions lead to a threshold voltage (VT) difference of at least 1 V between the two FETs. Our FETs with Pd and Ni/Ti, respectively, showed 0.8 and −0.3 V for their VT values, while our inverter exhibited a desirable voltage transfer characteristics with voltage gain of over 15 during low voltage electrical gating.

A ZnO nanowire-based photo-inverter with pulse-induced fast recovery

We demonstrate a fast response photo-inverter comprised of one transparent gated ZnO nanowire field-effect transistor (FET) and one opaque FET respectively as the driver and load. Under ultraviolet (UV) light the transfer curve of the transparent gate FET shifts to the negative side and so does the voltage transfer curve (VTC) of the inverter. After termination of UV exposure the recovery of photo-induced current takes a long time in general. This persistent photoconductivity (PPC) is due to hole trapping on the surface of ZnO NWs. Here, we used a positive voltage short pulse after UV exposure, for the first time resolving the PPC issue in nanowire-based photo-detectors by accumulating electrons at the ZnO/dielectric interface. We found that a pulse duration as small as 200 ns was sufficient to reach a full recovery to the dark state from the UV induced state, realizing a fast UV detector with a voltage output.

Annealing-induced conductivity transition in ZnO nanowires for field-effect devices

We report on the fabrication of ZnO nanowire (NW) devices in which the NWs were annealed in air ambient for their conductivity change from conducting to semiconducting states. Ambient annealing at 600 °C effectively gained a good semiconducting state of ZnO NW. Either top- or bottom-gate NW field-effect transistors (FETs) with optimally annealed ZnO NW showed a high on/off current ratio of ∼106, while the NW FETs with the initially conducting NWs appeared to keep on-state only. Schottky diode with the annealed NW displayed an ideality factor of ∼1.51 along with an on/off ratio of ∼103.

Photoelectric probing of the interfacial trap density-of-states in ZnO nanowire field-effect transistors

We have fabricated transparent top-gate ZnO nanowire (NW) field effect transistors (FETs) on glass and measured their trap density-of-states (DOS) at the dielectric/ZnO NW interface with monochromatic photon beams during their operation. Our photon-probe method showed clear signatures of charge trap DOS at the interface, located near 2.3, 2.7, and 2.9 eV below the conduction band edge. The DOS information was utilized for the photo-detecting application of our transparent NW-FETs, which demonstrated fast and sensitive photo-detection of visible lights.

Evaluation of metal–nanowire electrical contacts by measuring contact end resistance

It is known, but often unappreciated, that the performance of nanowire (NW)-based electrical devices can be significantly affected by electrical contacts between electrodes and NWs, sometimes to the extent that it is really the contacts that determine the performance. To correctly understand and design NW device operation, it is thus important to carefully measure the contact resistance and evaluate the contact parameters, specific contact resistance and transfer length. A four-terminal pattern or a transmission line model (TLM) pattern has been widely used to measure contact resistance of NW devices and the TLM has been typically used to extract contact parameters of NW devices. However, the conventional method assumes that the electrical properties of semiconducting NW regions covered by a metal are not changed after electrode formation. In this study, we report that the conventional methods for contact evaluation can give rise to considerable errors because of an altered property of the NW under the electrodes. We demonstrate that more correct contact resistance can be measured from the TLM pattern rather than the four-terminal pattern and correct contact parameters including the effects of changed NW properties under electrodes can be evaluated by using the contact end resistance measurement method.

Fabrication of vertical GaN/InGaN heterostructure nanowires using Ni-Au bi-metal catalysts.

We have fabricated the vertically aligned coaxial or longitudinal heterostructure GaN/InGaN nanowires. The GaN nanowires are first vertically grown by vapor–liquid-solid mechanism using Au/Ni bi-metal catalysts. The GaN nanowires are single crystal grown in the [0001] direction, with a length and diameter of 1 to 10 μm and 100 nm, respectively. The vertical GaN/InGaN coaxial heterostructure nanowires (COHN) are then fabricated by the subsequent deposition of 2 nm of InxGa1-xN shell on the surface of GaN nanowires. The vertical GaN/InGaN longitudinal heterostructure nanowires (LOHN) are also fabricated by subsequent growth of an InGaN layer on the vertically aligned GaN nanowires using the catalyst. The photoluminescence from the COHN and LOHN indicates that the optical properties of GaN nanowires can be tuned by the formation of a coaxial or longitudinal InGaN layer. Our study demonstrates that the bi-metal catalysts are useful for growing vertical as well as heterostructure GaN nanowires. These vertically aligned GaN/InGaN heterostructure nanowires may be useful for the development of high-performance optoelectronic devices.